Among other beneficial characteristics, carbon materials are known for their thermal conductive properties. For instance, carbon nanotubes, with individual nanotube thermal conductivity of 3,000 W/mK measured experimentally and up to 6,600 W/mK predicted from theoretical calculations, have generated much excitement with regard to their potential use in forming polymeric nanocomposites with ultrahigh thermal conductivities. So far, however, such nanocomposites have not been produced.
There has also been interest in the use of exfoliated graphite in forming thermally conductive nanocomposites. For example, Drzal and coworkers reported that graphite could be intercalated through chemical oxidation treatment and then rapidly exfoliated at higher temperature and that the exfoliated graphite could be dispersed into polymeric matrices including nylons and polyethylene for composites of enhanced thermal conductivities (see, e.g., Fukushima, et al., J. Therm. Anal. Cal. 85, 235-238 (2006); Kalaitzidou, et al., Carbon, 45, 1446-1452 (2007)). In this research, it was found that the thermal conductivity of a composite increased almost linearly with the graphite loading, up to 4.1 W m−1K−1 in a composite of nylon 6 with 20 vol % exfoliated graphite (compared to only 0.25 W m−1K−1 for the blank polymer).
Similarly, Haddon and coworkers processed natural graphite flakes into “graphite nanoplatelets” by initial treatment with a mixture of concentrated sulfuric acid and nitric acid for intercalation followed by exfoliation through thermal shock on rapid exposure of the intercalated graphite to various high temperatures in nitrogen (Yu, et al., J. Phys. Chem. C 111, 7565-7569 (2007)). The graphite nanoplatelets (GNPs) thus obtained were dispersed through a post-processing treatment for the fabrication of composites with epoxy. These GNP-epoxy composites were found to have thermal conductivities up to 6.44 W m−1K−1 at 25 vol % GNP loading, considerably higher than that of the blank epoxy.
U.S. Pat. No. 7,071,258 to Jang, et al. discloses a method for forming nano-scaled graphene plates that includes partially or fully carbonizing a precursor polymer or heat-treating petroleum or coal tar pitch to produce polymeric carbon containing micron- and/or nanometer-scaled graphite crystallites, exfoliating the thus formed graphite crystallites, and then subjecting the polymeric carbon containing exfoliated graphite crystallites to a mechanical attrition treatment such as ball milling.
U.S. Patent Application Publication No. 2004/0127621 to Drzal, et al. discloses a method for forming graphite nanoplatelets from expanded graphite as well as composites and products produced therefrom. The method of expanding the graphite is by microwaves or other radiofrequency wave treatment of intercalated graphite. Following expansion, the graphite is then crushed to a size of 200 microns or less in size. The expanded graphite can be used in forming polymer composites.
Connell and coworkers compared different nanoscale carbon fillers including multiple-walled carbon nanotubes (MWNTs), vapor-grown nanofibers, and commercially available expanded graphite in Ultem™ 1000 resin for enhanced thermal conductivities in extruded composite ribbons (Ghose, et al., High Performance Polymers 18, 961-977 (2006)). The expanded graphite was found to be more effective than the other fillers, with the in-plane thermal conductivity reaching 6.7 W m−1K−1 in the ribbon sample containing 40 wt % graphite.
While the above describe improvements in the art, room for additional improvement exists. What are needed in the art are relatively simple, efficient and inexpensive methods for preparing carbon-based fillers as may be used in forming highly thermal conductive composites.